No, you're not sandbagging!
Your 150 average as a Tour Pro of L59 gives away that you play on mobile and haven't found the path to ranked strokeplay games yet. On PC, their scores are used almost exclusively to govern a player's way through the tiers by means of the average. On mobile, tiers are also connected to the levels and rightly brought you to your spot.
To play 9 or 18 hole strokeplay, you have to chose "courses" and select a course before selecting the kind of game that you want to play. Mind you that strokeplay isn't available on all courses.
Feel invited to come back for further questions...